Carrier augmentation circuit



May 19, 1936f w. s. BARDEN CARRIER AUGMENTATION CIRCUIT Filed NOV. 26,1954 All S S 1NVN`TOR WILUAM S. BARDENv BY iff? wv-IA., ATTORNEY @55k bl m ...I

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Patented May 19, 1936 UNITED STATES.v

2,041,040 CARRIER AUGMENTATION CIRCUIT William Stoddard Barden,

Grasmere, Staten Island, N. Y., assgnor to Radio Corporation of America,a corporation of Delaware Application November 26, 1934, Serial No.754,713

9 Claims.

My present invention relates to signal carrier selection networks, andmore particularly to an improved method of, and means for, selecting lside band modulated carrier energy with carrier augmentation.

Intelligence modulated radio frequency signals transmitted to a receiverby radiation through space are subject to different types of fadingphenomena. To compensate for amplitude fading, there has been providedin the receiver an automatic volume, or gain, control network whichfunctions to maintain the signal input to the demodulator of thereceiver substantially constant over a predetermined range` of collectedlmodulated signal intensities. However, it has been recognized for sometime that an additional type of fading occurs during the receptionof'radiated signals; and this type of fading is particularly troublesomein the reception of signals radiated from points located a relativelygreat distance from the location of the receiver.

This latter type of fading, often called selective fading will occurregardless of the degree of selectivity of the selector network, and inspite of the use of an automatic gain control network. It has been foundthat when a transmitter radiates a signal comprising a carrier and bothside bands, and the distance between the transmitter and receiver isrelatively great, various effects in space may bring about relativephase shifts among the radio frequency components, or selectiveattenuation among the radio frequency components, of the radiated signalmay occur.

The desired carrier may fall to zero, or to a small value closerto zero;on the contrary, it may increase, and it has beenrfound that both theseeffects can occur at the receiving antenna. Furthermore, it has beenfound that the carrier may undergo a 90" phase shift with respect to itsproper self, and any side band component may do likewise. As aconsequence it has been found that the loud speaker output in typicalhighly selective receivers may become badly distorted, or greatlydiminished from time to time during the reception of side band modulated'carrier signals from transmitters located a relatively great dis tancefrom a given receiver.

Many arrangements have been proposed in the past to minimize the effectof `selective fading. Many of these proposed circuits and arrange-'ments of the prior art have proven to be unconomical, in many casesthey have been found to be complicated in operation and construction.

(Cl. Z50-20) Accordingly, it may be stated that itis one of the mainobjects of my present invention to pro' vide an improved and simplifiedcircuit arrangement for substantially minimizing the effects ofselective fading during the reception of side band modulated carrierenergy, the arrangement em-f ployed essentially comprising a sharplyselectiveV circuit, sharply tuned to the frequency of Vthe de siredcarrier, arrangedv in series with an imped` ance, and thecombined-seriesarrangement be, ing disposed in the signal transmission networkk of thereceiver in such a manner that carrier augmentation, or exaltation, isobtained whereby selective attenuation of the side bands of the car-.rier is compensated for by the carrier' augmenta,.-l tion effect. lAnother important object of the invention is' to provide in a radioreceiver successive devices for minimizing selective fading effects atthe receiver due to selective attenuation and relative phase shiftsamong the radio frequency components of the received signals, one of thedevices used in the receiver functioning to augment the carrier withrespect to its side bands suiciently to compensate for the selectiveattenuation effect, and the other of the devices functioning tosubstantially reject one of the side bands withl the result that therelative phase shift effect is substantially compensated for. v

Still another object of the invention is to provide in a superheterodynereceiver, adapted to receive side band modulated carrier signals fromrelatively remote points, simple and effective devices for materiallydecreasing the effects of radio frequency distortion in space upon thedesired signals, which effects may be due to relative' phase shifts andselective attenuation among the radio frequency components of thedesired signals, the superheterodyne receiver including be-Y tween thefirst and second detectors, devices f or substantially rejecting one ofthe side bands of. the received signals and increasing the carrierstrength relative to the strength of the remain# ing side band. A Andstill other objects of the invention are to improve generally theeiciency and simplicity ofradio receivers utilized for receivingmodulated signals from distant stations, and more especially to providesuch receivers with devices to mini` mize the effects of selectivefading, and which devices are reliable in operation, and economical.` lyconstructed and assembled in a radio receiver. The novel features whichI believe to be char-' acteristic of my invention vare set forth inparticularity in the appended claims, the inventionl itself, however, asto both Vits organization l reference.l to the following descriptiontaken inY Y superheterodyne type. Athat this type of receiver is chosento embody the and method ofY operation will'best be understood byconnection with the drawing in which I'have indicated diagrammatically acircuit organization whereby myinvention may be carried into effect.

Fig. Y1 schematically shows a. radior'rreceiverr` embodying the present"invention,

Fig. 2 graphically shows the operation of the invention. f

, Referring now to the accompanying drawing, itrwill be observed thatthercircuit'diagram in Fig; 1 schematically designates a receiver of theIt is lto be understood invention because the carrier augmentationnetwork employs a piezo-electric crystal, as an element'thereof, andsince the intermediate frequency amplifier network of a superheterodynereceiverk remains ixedlyV tuned to the operating intermediate frequency,4the piezo-electric crystalV Yelement may be'empl'oyed therein toadvantage.

' ThesuperheterOdyne receiver shown Yin `Elig. 1,

inrgeneral, comprises theconventional and wellknown networks. VThesignal collector' may consist of the usual grounded antennaY circuit;one or more tunable stages ofradio frequency ampli'-fication,;designated at |,omay be coupled to the signal collector A.YvThe frequency'changerfnet- 'Y work comprises Ya, first detector 2 and alocal oscillator 3,'and the output of Vthe rstfdetector -is impressed`upon an-intermediate frequency amplifier 4.A Y Y The tuner device ofthe. receiver is conven- Y Ytionally represented by the variablecondenser representations 5 and 6. Itis to be understood vthat vtheArepresentation 5 designates the variable tuning condensers usuallyemployed to tune the input circuits of the radio'frequencyamplifier, orampliers, fand the rst detector; therepresentation EiV designates thevariable tuning con- Y denser commonly utilized to'tune the localoscillator'net'work. The various variable condensers are, of course,uni-controlled, and any'well known device is utilized in the localoscillator network to maintain the local oscillation frequency at avalue which differs fromY the desired signal lfrequency by thevoperating intermediate frequency. Such devices for maintaining asubstantially constant intermediate frequency difference be- Ytween thesignal circuits of the local oscillator circuit Vare well known to thoseskilled in the art, and `need not be explained any further;`Furthermore, it is Yto be clearly understood that the networks 2 and ,3may be combined in a. single cir- Vcuit arrangement; that is to say, asingle tube may be employed to perform the functions of generation of'local oscillations and production of the intermediate frequency. Y Y

The-intermediate frequency energy amplified Y in the network 4 istransmited to the second de- Ytector, or demodulator, 1 through Yatransmission network embodying the present invention. The Y demodulatedsignalenergy is then fed toV an audio frequency network,rand thelattermay comprise one or more stagesof audio lfrequency `amplifica-vtion followed by a reproducer; Thetransmission network between theamplifier 4 and the second detector 'I comprisesra pair ofiintermediateVfre- Y quency amplifers'arranged in` cascade, there be Y ving disposedbetween the' amplifier 8 and the amplifier 9 anetwork forproducingcarrier augo, mentation with respect'toi the side bands, while.

between the amplier's :and the second detector 1, there is utilized a,Vnetwork for substantially rejecting one of the side bands Vof thesignal energy. transmitted from the amplifier/Sito the Y demodulator 1.

The amplifier 8 may be ofV any Ywell Yknown type, and is shown by way ofexampleas a screen grid tube. The input electrodes of amplifier 8 arecoupled to the anode circuit of lamplifier 4 through a coupling net workM1 which may comprise a pair of coupled circuits tuned to the oper-Vating intermediate frequency, and having the Vconstants Ythereof chosento provide a band pass characteristic. Thus, there is impressed uponVamplifier 8 the desired signal carrier, now at intermediate frequency,and its associated modulation side bands. As pointed out heretofore,vwhen the receiver is utilized to receive side band modulated signalsfrom a relatively remote point various causes in space may bring aboutrelative phase shifts among the radio frequency components, .orselective'attenuation among the components. YThe carrier amplitude mayfall sube stantially to zero with respect to the'modulation side bands;or the carrier may undergo a phase shiftwith respect to its properself.V 'Ihis @mayroccur whether the'received signalsare in the broadcastband, 500 to 1500.ki`locycles,1or

whether the received Ysignals arein the Vshort-V wave band, 8 to 23megacycles.'Y The selectivefading phenomena referred to willV resultVvin the loud speaker output becoming badly distorted, 'or Vgreatlydiminished, from vtime to time.

Additionally, the eiect of selective fading willV occur eventhough theselective network preceding the secondjV detector is sharp. The use ofan automatic vvolume control network in the, receiver is ofVsubstantially no` aid in overcoming the effect .of selective fading. Inother words, the receiver shown in Fig. 1 is to be understood asincluding an automatic volume controlv network, usually embodying arectifier supplied with intermediate frequency energy from a pointpreceding the second detector 'l and feeding this rectified directcurrent energy'as bias to the networks I, V2 and 4. Such a network iswell known to those skilled in the art, and isnot shown on the drawingin order to preserve simplicity of disclosure.

The important fact is that such an automatic gain control arrangement isof little use in overcoming the effects of selective fading at thereceiver, since the Vconventional automatic gain control arrangementdoes not operate to discriminate between attenuation of the carrier, orphase shift thereof, with respect to the associated side' bands. y YAccordingly, there Yis disposed in the anode Ycircuit of amplifier '8 asimple, but highly effective, network for overcoming the effect of theattenuation of thencarrier with respect to its side bands. The anodeoftube 8 is connected to the high alternating potential side of the tuned.

circuit L-C through a condenser I0. The anode is further connectedrtofasource of energizing direct current voltage B through a resistor Ilwhich may have amagnitu'de of about 5000 ohms. A resistor 1:is'arrang'ed inseries with the tuned circuit L-C and this resistor mayhave a mag- Y Vanimee ofV about 30,000 ohms; the 10W potential sides ofresistors l l and r being connected by a condenser I2. The coil L mayhave -a magnitude Y of about 20 millihenries, and the capacity ofcondenser CY is chosen tohave' a value such that with thefvalue of thecapacity existing between the plates of thecrystal. holder, it tunestheI coil Lto the operating intermediate frequency.

for present receivers to utilize an intermediate frequency of 450kilocycles. The invention does not depend in any way upon the value ofthe intermediate frequency chosen, nor upon the specific short-wavefrequency which it is desired to be received.

In shunt with the coil L, and with the condenser C, there is disposed apiezo-electric crystal device I3, and those skilled in the art willreadily Y appreciate that such a device customarily comprises a quartzcrystal disposed between metallic holder plates, and the crystal beingdesigned to be resonant to the operating intermediate frequency. Theinput electrodes of amplifier 9 are connected across the resistor r,while the output of amplifier 9 is transmitted to the demodulator 1through a band pass filter which is designed to substantially reject oneof the side bands of the intermediate frequency energy transmitted tothe demodulator.

In explaining the operation of the network included between amplifiers 8and 9, reference is made to the solid line curve shown in Fig. 2 whichrepresents the resonance curve characteristic of the network. It will beobserved from this solid line curve that the effect of the network is togreatly attenuate `the side bands of the intermediate frequency energywith respect to the carrier. Such a characteristic is secured by virtueof the fact that the coil L neutralizes, or tunes with, the crystalholder capacity so that at crystal resonance, the operating intermediatefrequency, there appears across points a, b, an equivalent seriesresistance of the crystal I3. For a crystal actually utilized, thisresistance was approximately 10,000 ohms. Thus, at crystal resonance thehigh resistance of the tuned circuit L-C, which was about one megohm,was nearly shorted by the 10,000 ohms resistance of the crystal deviceI3. As a consequence nearly all of the signal voltage was applied acrossthe resistor r connected in series with the crystal device I3 and havinga magnitude of 30,000 ohms.

However, o crystal resonance, the 10,000 ohms is removed by virtue ofthe crystal impedance rising greatly. Hence, 01T crystal resonance thecircuit L-C is a simple wave trap, and prevents substantially 95% of theside band voltage from being applied across resistor 1'. As a result,across the resistor 1' all of the carrier energy is developed, and onlyabout 5% of the associated side bands of the intermediate frequencyenergy. In other words, the action of the crystal device I3 is toautomatically reduce the signal energy voltage developed across resistorr as the frequency of the signal energy departs from the operatingintermediate frequency, which is also crystal frequency. The solid linecharacteristic in Fig. 2 demonstrates, furthermore, that there has beena relative carrier augmentation of the intermediate frequency energy.That is to say, the amplitude of the carrier has been exalted withrespect to the amplitude of the side bands.

As a consequence of the network inserted between amplifiers 8 and 9 theeffect of selective fading, wherein the amplitude of the carrier isgreatly diminished with respect to the side bands, on all signalscollected at antenna A is substantially minimized. The network disposedbetween amplifiers 8 and 9 functions to produce the reverse effect ofthe selective fading, and therefore there is transmittedfrom theamplifier 9`to the demodulator 'l signal energy wherein the amplitude ofthe carrier with respect to the side bands is'- substantially unalteredas compared to the same relation at the transmitter.

When, however, there also occurs in the radiated signals a phase shiftamong the radio frequency components, I have found that the utilizationof a band pass filter, conventionally designated by the referencenumeral 20, designed to substantially reject one of the side bands,greatly minimizes the phase shift effect. The band pass filter 20 isdesigned to change the solid line characteristic of Fig. 2 to the dottedline characteristic. The latter shows a substantial rejection of one o-fthe side bands.

Without entering into any theoretical considerations, it can bedemonstrated that the effects of carrier phase shift becomes of almostno consequence, and a carrier fade of ten to one becomes of no practicalconsequence when the demodulator 'I is a linear detector. Those skilledin the art will readily appreciate the manner of designing the band passfilter 20 so as to reject one of the side bands of the intermediatefrequency energy. Merely by way of illustration, it is pointed out thatthe design of such sharply tuned band pass iilters is described by G. A.Campbell in U. S. Patent 1,227,113 of May 22nd, 1917. For the purpose ofenhancing the dotted line characteristic shown in Fig. 2, it may bepreferable to employ an operating intermediate frequency of 50 or 75kilocycles.

The demodulator 1, when employing linear detection, and preceded by thecompensating devices described, materially decreases the eiects of radiofrequency distortion in space on desired signals. Any desired type oflinear detector may be utilized, and by way of illustration it ispointed out that a diode rectifier circuit can be employed because ofthe considerable intermediate frequency amplification preceding thedemodulator network. It will therefore be seen that there has beenprovided between the demodulator and the converter network anintermediate frequency amplifier which includes devices for materiallydecreasing the effects of radiov frequency distortion, or selectivefading, in space upon thel desired signal energy by rejecting one of theside bands of the intermediate frequency energy and increasing thecarrier strength relative to the strength of the remaining side band.

It is to be clearly understood that the side band rejection step mayprecedev the carrier augmentation step, and furthermore that the carrieraugmentation network may be disposed at ,any point between the converternetwork and the demodulator 1. When utilizing the receiver for receptionof signals ordinarily not subject to selective fading effects, as inlocal reception of broadcast signals, a switch 2| may be closed toshort-circuit the crystal device I3 and remove it from the circuit.

While I have indicated and described a system for carrying my inventioninto effect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular organization shownand described, but that many modifications may be made Without departingfrom the scope of my invention as set forth in the appended claims.

What I claim is:

1. A method of operating a radio receiver of the super-heterodyne typewhich includes collecting side band modulated carrier energy radiatedfrom a transmitter relatively remote from the receiver, converting thecollected energy to in- VVtermediate frequency energy, augmenting` Yther'amplitude of the carrier frequency of the intermediate frequencyenergy with respect to its ,asso-- ciatedside bands,substantially'rejecting one of the side bands, and subjecting theresulting energy to Vlinearrdetection. Y

' Y V2. A method of operatingV a radio` receiver of the superheterodynetype which includes collecting side band modulated carrier energyradiated from a transmitter relatively remote from the receiver,converting: the collected energy 'to intermediate Vfrequency energy,augmenting the amplitude 20Y ,ceiver to receive side band modulatedcarrier enof thev carrier frequency of Ythe intermediateY frequencyenergy with respect to its associated v side bands, in such a mannerthat the ratio of the carrier amplitude to the side band amplitude is ofthe order of nine to one, substantially rejecting one of the side bands,and subjecting the resulting energy to linear detection. t Y

3. A method of operating Ya superheterodyne reergyffrom a transmitterrelatively distant from the receiver in'such'a manner asY tosubstantially minimize the effects ofY selective fading on the signalsradiated. from the transmitter, which consists in reducing the carrierfrequency of the collected modulated carrier energy to aidesired inftermediate frequency of tl1e-order of 50 toV 450 kilocycle's, rejectingcnenof the side bands of Vthej intermediate frequency energy andincreasing the carrier amplitude relative to the strength of theremaining side band, and finally subjecting the resultant energy tolinear detection.V

4; In a superheterodyne receiver of the type including a Vconverternetwork having an output circuit tuned to an operating intermediatefrequency and a demodulator of the linear detection type, an impedanceof relatively low resistance arranged in series with the said tunedoutput circuit, means for coupling the demodulator input `across thesaid impedance, and a Vpiezoelectric device, tuned to the intermediatefrequency, connected across the tuned output circuit in such a mannerthat signal energyfoi"Y a. fre-VA quency diiTerent from the intermediate`frequency is greatly attenuated andprevented from appear'-Y ing acrosssaid impedance.V Y Y Y 5. In a system as dened in claim 4, saidpiezoelectric device having a resistive impedance substantially lowerthan the resistance of said series impedance. v Y Y 6. In a system asdescribed in claim 4, said coupling means between said series impedanceand said demodulator including a network constructedV to substantiallyVreject one of the Vside bands of Y the intermediate frequency energy.

7. In a superlieterodyne receiver of the type provided with an`intermeradi'ate frequency ampli-- fier followed by a demodulatoroperating by virtue of linear detection, aV resonant circuit including acoil and condenser arranged in parallel in the anode circuit of theamplifier, said resonant circuit being tuned Vto the intermediatefrequency, a resistor having :a substantiallyrlow resistance arranged inseries with the resonant circuit,l an

intermediate frequency coupling network con- Ynesting the seriesresistance to the said demodulater so as. to impress -uponthe'dem'o'dulator in-V 25 Ytermediate frequency energy developed acrossthe said series resistor, andaV piezo-electric crystalconnected-in shuntwith'sai'd4 coil andcon'densen said crystal beingl sharply tunedto theintermediatev frequency and having its constants chosen to substantiallyinhibit the development across said series resistor of the side bandfrequencies of the intermediate frequency energy. 8. In a system asdefined in claim '7, means for rendering said crystal inoperative.

9. In a system as defined in claim 7, a band pass filter disposed insaid intermediate frequency cou plingnetwork and having its constantschosen whereby one of the side bands of the intermediate frequencyenergy transmitted to said demodulator is substantially rejected. Y

WILLIAM sToDDARD BARDEN.

